You will need to make sure the prefs value for the sent_mail_folder
entry is empty in your preferences backend. Or try switching to
another (valid) sent mail folder and then switching back to the
default option.

Yes I switched to the valid 'Sent' folder (and had no errors) and then
back to 'Use Default Value'. I verified the git pull updated the
files, and I deleted all the static cache files too.

Oddly enough if I select 'None' and save it, it is successful but it
did not change the sent folder, it is still what is was previously.

You will need to make sure the prefs value for the sent_mail_folder
entry is empty in your preferences backend. Or try switching to
another (valid) sent mail folder and then switching back to the
default option.